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 Post subject: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Thu 23, 2021 8:40 pm 
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Joined: Jan Fri 20, 2017 9:36 pm
Posts: 203
Location: Centuria, Wisconsin
Yesterday someone dropped of this radio. He heard that I worked on them and thought maybe I could use it for parts. It was being tossed as the cabinet was not in very good shape, so he retrieved it and brought it over.
I was trying to find information on this particular model, but Riders jumps over this one. On examining it, I noticed that it does not have a tuning capacitor. The tuning dial string is connected to movable slugs in 2 coils, presumably the antenna and oscillator coils? There are 2 trimmer capacitors that I presume are for alignment. The slug in the antenna coil is stuck, so the string broke. There is not much to the radio so I may try to get it operational just for the experience of working on this unique tuning system. The IF, field, and output transformer have continuity so the rest of the parts are minimal. I don't think I can "restore" the cabinet.
Another thing I noticed was an interesting tube in the 12SA7 socket. It is a Delco brand but I cannot read the markings. The glass tube is only about 1 inch tall! I have not run across one like this before.

John


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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Thu 23, 2021 8:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Rochester NY USA
That looks like the glass version of an 0Z4 - probably not the greatest converter tube... but hey - fits the socket!

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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Thu 23, 2021 9:42 pm 
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Joined: Jan Tue 31, 2012 1:55 am
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Location: Mountains of Mourne. Ireland.
Service data from Rider 1950 attached below...

Greg.


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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Thu 23, 2021 9:54 pm 
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Joined: Jan Fri 20, 2017 9:36 pm
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Location: Centuria, Wisconsin
Greg: Thanks for the schematics. I was looking in Riders for 1942, as that was the listed year of manufacture. It does show the variable inductors instead of the variable capacitor as main tuning. Should be interesting to try to get it to work.

John


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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Fri 24, 2021 12:43 pm 
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Location: Ohio 45177
I wonder what motivated makers to use the variable inductance tuning when it seems more complicated that buying an off the shelf tuning capacitor, for consumer radios. I can't imagine that it was a cost savings, with the extra hardware and alignment steps. As you could buy the standard cap, and a standard oscillator coil and antenna loop and not have to worry about the engineering. Unless there was a shortage, which seems unlikely due to the majority not adopting that.

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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Fri 24, 2021 1:52 pm 
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There were parts and tube shortages between 1946 and 1952. Just as things started to ramp up after WWII, the Korean War came along.

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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Fri 24, 2021 1:54 pm 
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Location: Mt. Prospect, IL USA
I have a Firestone 4A-21X made by Stewart-Warner. It has the traditional tuning gang cap. But there was also a 4A-21 that was the same, except it used slug tuning. Perhaps after the war, there was a shortage of new tuning caps. And alternative designs were made using slug tuning inductors, to make production possible. Zenith was forced to make their own speakers and record changers after the war due to shortages. (And every postwar Zenith phono motor in my collection ended up frying itself...). The dial stringing was even the same between the 4A-21 and 4A-21X.


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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Sun 26, 2021 6:24 pm 
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Joined: Jan Fri 20, 2017 9:36 pm
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Location: Centuria, Wisconsin
Not a very good day! I was trying to free the ferrite slug in the above chassis antenna coil, and the slug in the oscillator coil under the chassis fell out. Of course, it fell on the floor and broke into 3 pieces. I am sure that a replacement slug would be almost impossible to find, so I am considering trying to super glue it back together. Even if I could glue it straight enough to slide in the coil, how would the glue affect the tuning? I am sure it would affect the overall permeability of the slug?

Each coil is in a holder that has slotted holes for mounting to the chassis. I would suspect that part of the alignment would include positioning the holder for proper tracking. With a glued together slug, it may be difficult to get the correct tracking, if it even works. After all this, I still have not been able to completely free the antenna coil slug. What can go wrong next!

John


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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Sun 26, 2021 11:01 pm 
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Location: Sterling Heights, Michigan 48314, USA
I wonder if you could craft a new slug….possibly from a cannibalized ferrite antenna rod ?
I suppose it would be pure luck to find one with the correct diameter……

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 Post subject: Re: Firestone S-7402-8, lookin for schematic
PostPosted: Sep Sun 26, 2021 11:23 pm 
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Location: Sterling Heights, Michigan 48314, USA
Repairing broken ferrite…
Maybe useful in your case ……
But super glue instead of epoxy?
https://www.antiqueradios.com/forums/viewtopic.php?f=10&t=105051

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